With increase in frequency of a.c. supply, the impedance of an L-C-R series circuit (a) remains constant. (b) increases. (c) decreases. (d) decreases at first, becomes minimum and then increases.

Answer: (d). f₀fZZ = R Z = √(R² + (XL - XC)²) with XL = ω L rising and XC = (1)/(ω C) falling as frequency rises. Below resonance XC > XL and the gap shrinks, so Z decreases. At resonance XL = XC, so Z = R (minimum); above it XL - XC grows, so Z increases. So the answer is (d) decreases at first, becomes minimum and then increases.

With increase in frequency of a.c. supply, the impedance of an L-C-R… | MHT CET 2023 (13 May, Shift 1)

Q61MHT CET 2023 · 13 MAY · SHIFT 1Easy

Resonance, Bandwidth & Q Factor

With increase in frequency of a.c. supply, the impedance of an L-C-R series circuit

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Answer: (d)

$Z = \sqrt{R^2 + (X_L - X_C)^2}$ with $X_L = \omega L$ rising and $X_C = \frac{1}{\omega C}$ falling as frequency rises.
Below resonance $X_C > X_L$ and the gap shrinks, so $Z$ decreases.
At resonance $X_L = X_C$ , so $Z = R$ (minimum); above it $X_L - X_C$ grows, so $Z$ increases.
So the answer is (d) decreases at first, becomes minimum and then increases.

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Resonance, Bandwidth & Q Factor question from MHT CET 2023 (13 May, Shift 1)

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